This research program addresses basic questions related to perception of auditory information and recognition of speech by individuals with normal hearing and cochlear hearing loss. Experiments are designed to test hypotheses within the framework of the active process of the cochlea, which is responsible for many fundamental properties of audition. It is well established that declines in cochlear function result in higher detection thresholds and broader tuning at lower levels. However, beyond these general effects, a thorough understanding of the negative consequences of a reduced active mechanism is lacking due to the limited knowledge of nonlinear effects both for complex sounds such as speech, and also for individuals with mild-to-moderate cochlear hearing loss. A comprehensive series of experiments is planned within two broad aims to address key questions concerning the effects of basilar-membrane nonlinearities on auditory perception and recognition of speech. Each of these aims is driven by hypotheses that examine auditory function for simple and complex sounds within a framework that provides physiologically-appropriate rationales for both the normal and impaired auditory system. Aim 1 assesses temporal masking, growth of masking, and suppression for tonal and speech signals over a wide range of levels, and changes in tuning with increasing level, to test the hypothesis that nonlinearities in the basilar-membrane response underlie improvements in speech recognition in noise at moderate levels, declines at higher levels, and diminished performance where gain is reduced by cochlear impairment. Aim 2 measures detection and recognition of sounds with fluctuating envelopes and masking of tones and speech in fluctuating maskers, to test the hypothesis that nonlinearities in the basilar-membrane response underlie changes in the temporal envelopes of speech and noises with increasing level, the benefit derived from masker fluctuations, and diminished performance where gain is reduced by cochlear impairment. A long-term goal is to discover the bases and means for reducing the detrimental effects of cochlear hearing loss on the perception of speech. A better understanding of these effects is essential if individuals with cochlear hearing loss are to achieve maximum benefit from amplified speech in adverse listening conditions.